Shape selective alkylation of toluene is one of highly valuable reactions in the industrial development because it can convert toluene to para-xylene having higher practical value, and can make the reaction system selectively produce para-xylene. ZSM-5 molecular sieves are currently used in most of relevant patents as the reaction body. ZSM-5 zeolites have a three-dimensional pore channel system consisting of 10-membered silicon oxygen rings. The pore diameter of ZMS-5 zeolites enables rapid diffusion of para-xylene having a molecular diameter of 0.63 nm, while ortho-xylene and meta-xylene having a molecular diameter of 0.69 nm have a much lower diffusion coefficient. In the toluene shape selective alkylation reaction system, the diffusion coefficients of various substances in ZSM-5 pore channels are in a sequence of toluene >ethyl benzene≈para-xylene>ortho-xylene≈meta-xylene. Such a fact means the possibility of the shape selection for the shape selective alkylation reaction of toluene. That is to say, the para-xylene isomer content greatly higher than thermodynamic equilibrium concentration in the xylene products can be obtained. However, the isomerization reaction rate at the external surface acidic position that has no selectivity to the enriched para-product diffused from the pore channel is far greater than the disproportionation rate, such that the final products reach a balanced composition soon. Although studies show that some certain selectivity can also be obtained by using ZSM-5 having large crystal particle size under the conditions of high space velocity and low conversion rate, it has no practical value. In conclusion, it is necessary to modify the external surface of ZSM-5. Moreover, the silica/alumina ratio, crystal size and shape of ZSM-5 molecular sieves will directly have an effect on the surface modification effects of the molecular sieves, so as to determine the performance index of the catalysts to a great extent.
U.S. Pat Nos. 5,367,099 and 5,607,888 disclose preparing selective toluene alkylating catalysts by modification of ZSM-5 molecular sieve structure, i.e., decreasing the opening size of the pore, and shielding the acidic activity position of the external surface. ZSM-5 molecular sieves used in said U.S. patents are synthesized with the organic amine template agent system. The organic amine template agent has advantages of fast crystallization rate and broad range of the structural silica/alumina ratio. Although the toluene selective alkylating catalyst prepared from ZSM-5 molecular sieves crystallized and synthesized with organic amine template agent has better reaction performance, the organic amine template agent results in severe environmental pollution and higher recovery cost during the synthesis. In U.S. Pat. No. 6,486,373, inorganic amines are used as the template agent for the synthesis of ZSM-5 molecular sieves. However, the silica/alumina ratio of the surface of the synthesized molecular sieves is greatly different from that of the frame thereof. Upon modification, ZSM-5 molecular sieves synthesized thereby have worse reaction performance than ZSM-5 synthesized using organic amine template agent. The toluene conversion rate greatly decreases, while the para-selectivity of the modified catalysts sharply increases. That is to say, there is a so-called “inverse effect”. The present invention is provided to solve the problems, such as a great deal of synthesized waste water present in the application of many organic amine template agents, higher production cost, worse comprehensive performance of the catalyst upon the modification of the molecular sieves synthesized by using inorganic amine template agents.